This invention relates to a method and apparatus for manufacturing laminated tapes suitable for use in superconducting magnets.
As is well known, a magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnetic coils to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of ten minutes) to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible coil resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter called "MRI").
Considerable research and development efforts have been directed at eliminating the need for a boiling cryogen, such as liquid helium, and in providing superconducting magnets which maintain the magnetic state and do not "quench," or discontinue superconductivity. However, the differential thermal expansion and contraction between materials in superconducting magnets, which are cycled from ambient temperature to temperatures in the range of absolute zero (-270.degree. C.), and the extremely large magnetic forces provided, and utilized, in an MRI lead to conflicting characteristics required of the materials used in MRI tapes. In addition, the desired superconducting magnet material such as Nb.sub.3 Sn is often relatively brittle and difficult to handle in winding coils.
Improved materials for use in superconducting magnets and associated superconducting switches are disclosed in United States patent applications of Bu-Xin Xu, R. Jaykumar and John Wollan, entitled "Persistent Superconducting Switch for Conduction-Cooled Superconducting Magnet," serial number 001,692 filed Jan. 7, 1993, and the patent application of Bu-Xin Xu and John Wollan, entitled "Stable Flux-Jumping Resistant Superconducting Tape and Superconducting Magnet," application serial number 984,819 filed Dec. 3, 1992, both of which are assigned to the same assignee as the present invention. The subject patent application is particularly suitable for use in winding superconducting coils and superconducting switches of the type disclosed in the aforementioned copending patent applications.
However, the manufacture of laminated tape suitable for superconducting use involves the lamination of long lengths of different materials such as niobium tin (Nb.sub.3 Sn) and copper by applying pressure while the Nb.sub.3 Sn and copper foils are fed through a molten solder bath and pinched together. A persistent problem encountered in such a process is contamination at the pinch point by debris and copper particles or compounds in the solder bath. Also, the tension required for feeding the thin foils through the solder bath and pinch point is different than the tension required for proper spooling of the resultant laminated tape onto a takeup spool.
It is highly desirable to be able to clean the pinch point or compression mechanism without lifting the mechanism out of the solder bath and while the process continues in operation. In addition, it is important to provide a uniform and carefully controlled laminated tape through control of the pressure exerted on the foils at the pinch point. Still further, it is highly important that the feed mechanism be reliable, be able to handle the extremely thin foils involved, and that provision be included for ease of replacement and/or repair of components in order to decrease process down-time for repair or maintenance purposes.